This invention relates to emulsion pressure-sensitive adhesives for pressure-sensitive labels and the like.
Pressure-sensitive adhesives based on styrene-butadiene rubber (SBR) polymers and tackifying resins are widely used. They have the advantages of low cost and good adhesive properties. However, commercially available SBR pressure-sensitive adhesives have been solvent based. Such adhesives are applied to a substrate such as a label stock and the solvent is evaporated to leave behind a permanently tacky film. The solvents which are evaporated from the label structure during drying of the pressure-sensitive adhesive film can escape into the atmosphere and contribute to air pollution. Environmental regulations limiting the venting of solvents into the atmosphere are becoming increasingly stringent. In addition, cost of the solvents is increasing rapidly. For these reasons, the large scale use of solvent-based adhesives is becoming less and less attractive.
Pressure-sensitive adhesives which are applied as aqueous emulsions have found favor because the venting of solvents into the atmosphere is substantially eliminated. Polymers which perform well in solvent-based, pressure-sensitive adhesive systems do not necessarily perform well in emulsion systems. The majority of emulsion pressure-sensitive adhesives intended for applications such as adhesive labels and the like, have been based on acrylic polymer emulsions. Acrylic emulsion pressure-sensitive adhesives have excellent mechanical stability, i.e., they resist coagulation when subjected to high shear conditions, such as conditions prevailing in certain types of coating machinery. In addition, acrylic emulsion adhesives have excellent adhesive properties in the dry film state. The primary disadvantage of the acrylic emulsion adhesives is their relatively high cost.
SBR emulsions are significantly lower in cost than acrylic emulsions. Although SBR polymers perform well in solvent-based adhesive systems, they have not found commercial acceptance in emulsion-based systems. A variety of adhesive compositions containing carboxylated SBR emulsions have been described. For example, U.S. Pat. No. 3,966,661 to Feast et al describes the preparation of carboxylated SBR latexes and discloses their use in carpet backings and paper coating applications. However, the use of carboxylated SBR emulsion in combination with a tackifying resin emulsion in a pressure-sensitive adhesive is not disclosed.
U.S. Pat. No. 3,951,900 to Bath describes a fire-retardant adhesive, which includes a carboxylated SBR emulsion and gypsum powder and other ingredients. The adhesive disclosed is not pressure-sensitive adhesive, but rather must be dried to perform its adhesive function.
U.S. Pat. No. 3,920,600 to Ahranjian describes pressure-sensitive adhesives which contain a carboxylated neoprene latex and an emulsified tackifier resin. Pressure-sensitive adhesives containing carboxylated SBR emulsions are not described.
U.S. Pat. No. 4,036,804 to Herai et al describes adhesive compositions for bonding a rubber material to metal substrate, which contain an aqueous carboxylated SBR emulsion, a vulcanizing agent and a vulcanization accelerator. Pressure-sensitive adhesives are not described.
U.S. Pat. No. 3,341,392 to Potter describes the use of carboxylated SBR latexes for the lamination of sheets of fibrous materials such as paper and cardboard. The described adhesives contain a carbohydrate adhesive such as starch. Pressure-sensitive adhesives are not described.
Although carboxylated SBR emulsions have been commercially available for some time, it has not been heretofore appreciated that such emulsions can be combined with tackifying resin emulsions to produce a commercially satisfactory pressure-sensitive adhesive.